Glaciers in Chilean Patagonia tell a story of climate change
Slicing through the Andes mountains until they reach the saw-toothed fjords of southern Chile are a series of seventy glaciers that make up the Northern Patagonian Icefield—almost all of which are retreating. To understand their future, researchers are looking at their history to tease out how past climates affected these icy behemoths.
For the first time in northern Patagonia, scientists have reconstructed a glacier’s evolution over the past 5,400 years by studying the sediment it left behind and have uncovered a reason for their retreat.
“Glaciers fluctuate naturally,” says Sébastian Bertrand, a young Belgian geologist who just returned from his tenth trip to Patagonia. “What has become alarming in the recent decades is the accelerated rate of glacier retreat.” Most Patagonian glaciers are retreating—with the exception of Perito Moreno and Brüggen/Pio XI—and Bertrand thinks looking back in time will help explain how glaciers will adapt to climate change in the 21st century.
Bertrand, a postdoctoral fellow at Woods Hole Oceanographic Institution in Massachusetts and the University of Ghent in Belgium, specializes in paleoclimatology—the study of climate changes across time. He’s just published a study looking at sediments left behind by the Gaulas Glacier, a 75-square-mile floe that carves its way down 20 miles of Patagonian Andes in southern Chile.
For millennia, the glacier has dumped sediment into Golfo Elefantes, a glacial lake from which Bertrand took a sediment core during a research cruise in 2005. The 50-foot-long core is made up of 5,400 years of sediment, deposited in layers of sand, silt, and organic material carried to its final resting place by the Gualas Glacier.
“When the glacier advances it creates sediment particles by eroding the rocks underneath it,” Bertrand wrote in an email. “However, it’s when the glacier retreats, so when the ice melts, that most of this sediment is transported to the fjords by rivers of meltwater.” Analysis of the sediment layers and radiocarbon dating of the embedded organic material—tree remains and carbonate shells—revealed that the glacier had seen three major periods of advance in the last 5,400 years. These had occurred between 4,200 and 900 years ago during a time that geologists call the “Neoglaciation” period.
But with glacial advances came retreats, the most drastic of which has occurred in the last century. The Gualas Glacier has retreated 8.9 kilometers (5.5 miles) since 1898, including 2.8 kilometers in the last 25 years. These conclusions were gleaned from historical records that included aerial photos, satellite images, and observations made by German geographer Hans Steffen in 1898.
“Steffen was contracted in the mid 19th century by the Chilean government to explore the southern parts of the country and Chilean waters in general,” says Fernando Torrejón, a Chilean historian who coauthored the glacier study with Bertrand. Torrejón works at Chile’s EULA Center, a multidisciplinary institution that provides historical data to studies like Bertrand’s. “Everyone wants historical records because they establish a baseline at a time where there was little or no human intervention,” says Torrejón.
But was there a way of telling what environmental factors had caused these advances and retreats?
Glaciers expand and contract depending on a variety of factors, says Bertrand, like those tied to climate—temperature, precipitation, solar radiation—and geological aspects like morphology of the bedrock and the dynamics of ice behavior.
Bertrand wanted to see if the major fluctuations he found in the core sample could be tied to a specific cause. His team scrutinized historical sea surface temperatures and pollen records and found that the glacial fluctuations were driven by changes in precipitation. Pollen records are a proxy for precipitation: the higher the pollen count, the more precipitation there was that year. Bertrand’s study, published March 15th in the journal Climate of the Past, concludes that the rapid retreat of the glaciers in Chile’s Northern Patagonian Icefield is driven by a decrease in winter precipitation. On the other side of the Andes, however, temperature seems to be the driving force behind those glaciers’ fluctuations.
Bertrand and his collaborators have been collecting sediment and water samples from all over Patagonia for years, in an effort to improve our understanding of the effects of climate in the past. Compared to the northern hemisphere, Bertrand says, the southern hemisphere is relatively understudied when it comes to climate research.
Glacial fluctuations have deep implications for the availability of freshwater and sea-level rise. How they fit into the water cycle is crucial to understanding their future, while looking at their past holds answers to how they will respond to a changing environment.
Sébastien Bertrand is a Belgian scientist looking at past climates.
Photos courtesy of Sébastien Bertrand and Fernando Torrejón.
Article published in Spanish on Science Friday en Español.